Architecture in the smallis concerned with thearchitecture of individual programs. At this level, we areconcerned with the way that an individual program isdecomposed into components.

Architecture in the largeis concerned with thearchitecture of complex enterprise systems that includeother systems, programs, and program components.These enterprise systems are distributed over differentcomputers, which may be owned and managed bydifferent companies.

6

Chapter 6 Architectural design

Advantages of explicit architecture

Stakeholder communication

Architecture may be used as a focus of discussion by systemstakeholders.

System analysis

Means that analysis of whether the system can meet its non-functional requirements is possible.

because they lacksemantics, do not show the types of relationshipsbetween entities nor the visible properties of entities inthe architecture.

Depends on the use of architecturalmodels.The

requirements for model semantics depends on how themodels are used.

8

Chapter 6 Architectural design

Box and line diagrams

Very abstract-

they do not show the nature ofcomponent relationships nor the externally visibleproperties of the sub-systems.

However, useful for communication with stakeholdersand for project planning.

9

Chapter 6 Architectural design

Use of architectural models

As a way of facilitating discussion about the systemdesign

A high-level architectural view of a system is useful forcommunication with system stakeholders and project planningbecause it is not cluttered with detail. Stakeholders can relate toit and understand an abstract view of the system. They can thendiscuss the system as a whole without being confused by detail.

As a way of documenting an architecture that has beendesigned

The aim here is to produce a complete system model that showsthe different components in a system, their interfaces and theirconnections.

Chapter 6 Architectural design

10

Architectural design decisions

Architectural design is a creative process so the processdiffers depending on the type of system beingdeveloped.

However, a number of common decisions span alldesignprocesses and these decisions affect the non-functional characteristics of the system.

11

Chapter 6 Architectural design

Architectural design decisions

Is there a generic application architecture that can beused?

How will the system be distributed?

What architectural styles are appropriate?

What approach will be used to structure the system?

How will the system be decomposed into modules?

What control strategy should be used?

How will the architectural design be evaluated?

How should the architecture be documented?

12

Chapter 6 Architectural design

Architecture reuse

Systems in the same domain often have similararchitectures that reflect domain concepts.

Application product lines are built around a corearchitecture with variants that satisfy particular customerrequirements.

The architecture of a system may be designed aroundone of more architectural patterns or ‘styles’.

These capture the essence of an architecture and can beinstantiated in different ways.

Discussed later in this lecture.

13

Chapter 6 Architectural design

Architecture and system characteristics

Performance

Localise

critical operations andminimise

communications. Uselarge rather than fine-grain components.

Security

Use a layered architecture with critical assets in the inner layers.

Safety

Localise

safety-critical features in a small number of sub-systems.

Availability

Include redundant components and mechanisms for faulttolerance.

Maintainability

Use fine-grain, replaceable components.

14

Chapter 6 Architectural design

Architectural views

What views or perspectives are useful when designingand documenting a system’s architecture?

What notations should be used for describingarchitectural models?

Each architectural model only shows one view orperspective of the system.

It might show how a system is decomposed into modules, howthe run-time processes interact or the different ways in whichsystem components are distributed across a network. For bothdesign and documentation, you usually need to present multipleviews of the software architecture.

15

Chapter 6 Architectural design

4 + 1 view model of software architecture

A logical view, which shows the key abstractions in thesystem as objects or object classes.

A development view, which shows how the software isdecomposed for development.

A physical view, which shows the system hardware andhow software components are distributed across theprocessors in the system.

Related using use cases or scenarios (+1)

16

Chapter 6 Architectural design

Architectural patterns

Patterns are a means of representing, sharing andreusing knowledge.

An architectural pattern is a stylized description of gooddesign practice, which has been tried and tested indifferent environments.

Patterns should include information about when they areand when the are not useful.

Patterns may be represented using tabular and graphicaldescriptions.

17

Chapter 6 Architectural design

The Model-View-Controller (MVC) pattern

Name

MVC

(Model-View-Controller)

Description

Separates

presentation

and

interaction

from

the

system

data.

The

system

is

structured

into

three

logical

components

that

interact

with

each

other.

The

Model

component

manages

the

system

data

and

associated

operations

on

that

data.

The

View

component

defines

and

manages

how

the

data

is

presented

to

the

user.

The

Controller

component

manages

user

interaction

(e.g.,

key

presses,

mouse

clicks,

etc.)

and

passes

these

interactions

to

the

View

and

the

Model.

See

Figure

6.3.

Example

Figure

6.4

shows

the

architecture

of

a

web-based

application

system

organized

using

the

MVC

pattern.

When

used

Used

when

there

are

multiple

ways

to

view

and

interact

with

data.

Also

used

when

the

future

requirements

for

interaction

and

presentation

of

data

are

unknown.

Advantages

Allows

the

data

to

change

independently

of

its

representation

and

vice

versa.

Supports

presentation

of

the

same

data

in

different

ways

with

changes

made

in

one

representation

shown

in

all

of

them.

Disadvantages

Can

involve

additional

code

and

code

complexity

when

the

data

model

and

interactions

are

simple.

18

Chapter 6 Architectural design

The organization of the Model-View-Controller

19

Chapter 6 Architectural design

Web application architecture using the MVCpattern

20

Chapter 6 Architectural design

Layered architecture

Used to model the interfacing of sub-systems.

Organises the system into a set of layers (or abstractmachines) each of which provide a set of services.

Supports the incremental development of sub-systems indifferent layers. When a layer interface changes, only theadjacent layer is affected.

However, often artificial to structure systems in this way.

21

Chapter 6 Architectural design

The Layered architecture pattern

Name

Layered

architecture

Description

Organizes

the

system

into

layers

with

related

functionality

associated

with

each

layer.

A

layer

provides

services

to

the

layer

above

it

so

the

lowest-level

layers

represent

core

services

that

are

likely

to

be

used

throughout

the

system.

See

Figure

6.6.

Example

A

layered

model

of

a

system

for

sharing

copyright

documents

held

in

different

libraries,

as

shown

in

Figure

6.7.

When

used

Used

when

building

new

facilities

on

top

of

existing

systems;

when

the

development

is

spread

across

several

teams

with

each

team

responsibility

for

a

layer

of

functionality;

when

there

is

a

requirement

for

multi-level

security.

Advantages

Allows

replacement

of

entire

layers

so

long

as

the

interface

is

maintained.

Redundant

facilities

(e.g.,

authentication)

can

be

provided

in

each

layer

to

increase

the

dependability

of

the

system.

Disadvantages

In practice, providing a clean separation between layers is oftendifficult and a high-level layer may have to interact directly withlower-level layers rather than through the layer immediatelybelow it. Performance can be a problem because of multiplelevels of interpretation of a service request as it is processed ateach layer.

22

Chapter 6 Architectural design

A generic layered architecture

23

Chapter 6 Architectural design

The architecture of the LIBSYS system

24

Chapter 6 Architectural design

Key points

A software architecture is a description of how a softwaresystem is organized.

Architectural design decisions include decisions on thetype of application, the distribution of the system, thearchitectural styles to be used.

Architectures may be documented from several differentperspectives orviewssuch

as a conceptual view, alogical view, a process view, and a development view.

Architectural patterns are a means of reusing knowledgeabout generic system architectures. They describe thearchitecture, explain when it may be used and describeits advantages and disadvantages.

Chapter 6 Architectural design

25

Chapter 6–

Architectural Design

Lecture 2

26

Chapter 6 Architectural design

Repository architecture

Sub-systems must exchange data. This may be done intwo ways:

Shared data is held in a central database or repository and maybe accessed by all sub-systems;

Each sub-system maintains its own database and passes dataexplicitly to other sub-systems.

When large amounts of data are to be shared, therepository model of sharing is most commonlyused athis is an efficient data sharing mechanism.

27

Chapter 6 Architectural design

The Repository pattern

Name

Repository

Description

All

data

in

a

system

is

managed

in

a

central

repository

that

is

accessible

to

all

system

components.

Components

do

not

interact

directly,

only

through

the

repository.

Example

Figure

6.9

is

an

example

of

an

IDE

where

the

components

use

a

repository

of

system

design

information.

Each

software

tool

generates

information

which

is

then

available

for

use

by

other

tools.

When

used

You

should

use

this

pattern

when

you

have

a

system

in

which

large

volumes

of

information

are

generated

that

has

to

be

stored

for

a

long

time.

You

may

also

use

it

in

data-driven

systems

where

the

inclusion

of

data

in

the

repository

triggers

an

action

or

tool.

Advantages

Components

can

be

independent—they

do

not

need

to

know

of

the

existence

of

other

components.

Changes

made

by

one

component

can

be

propagated

to

all

components.

All

data

can

be

managed

consistently

(e.g.,

backups

done

at

the

same

time)

as

it

is

all

in

one

place.

Disadvantages

The

repository

is

a

single

point

of

failure

so

problems

in

the

repository

affect

the

whole

system.

May

be

inefficiencies

in

organizing

all

communication

through

the

repository.

Distributing

the

repository

across

several

computers

may

be

difficult.

28

Chapter 6 Architectural design

A repository architecture for an IDE

29

Chapter 6 Architectural design

Client-server

architecture

Distributed system model which shows how data andprocessing is distributed across a range of components.

Can be implemented on a single computer.

Set of stand-alone servers which provide specificservices such as printing, data management, etc.

Set of clients which call on these services.

Network which allows clients to access servers.

30

Chapter 6 Architectural design

The Client–server pattern

Name

Client-server

Description

In

a

client–server

architecture,

the

functionality

of

the

system

is

organized

into

services,

with

each

service

delivered

from

a

separate

server.

Clients

are

users

of

these

services

and

access

servers

to

make

use

of

them.

Example

Figure

6.11

is

an

example

of

a

film

and

video/DVD

library

organized

as

a

client–server

system.

When

used

Used

when

data

in

a

shared

database

has

to

be

accessed

from

a

range

of

locations.

Because

servers

can

be

replicated,

may

also

be

used

when

the

load

on

a

system

is

variable.

Advantages

The

principal

advantage

of

this

model

is

that

servers

can

be

distributed

across

a

network.

General

functionality

(e.g.,

a

printing

service)

can

be

available

to

all

clients

and

does

not

need

to

be

implemented

by

all

services.

Disadvantages

Each

service

is

a

single

point

of

failure

so

susceptible

to

denial

of

service

attacks

or

server

failure.

Performance

may

be

unpredictable

because

it

depends

on

the

network

as

well

as

the

system.

May

be

management

problems

if

servers

are

owned

by

different

organizations.

31

Chapter 6 Architectural design

A client–server architecture for a film library

32

Chapter 6 Architectural design

Pipe and filter architecture

Functional transformations process their inputs toproduce outputs.

May be referred to as a pipe and filter model (as in UNIXshell).

Variants of this approach are very common. Whentransformations are sequential, this is a batch sequentialmodel which is extensively used in data processingsystems.

Not really suitable for interactive systems.

33

Chapter 6 Architectural design

The pipe and filter pattern

Name

Pipe

and

filter

Description

The

processing

of

the

data

in

a

system

is

organized

so

that

each

processing

component

(filter)

is

discrete

and

carries

out

one

type

of

data

transformation.

The

data

flows

(as

in

a

pipe)

from

one

component

to

another

for

processing.

Example

Figure

6.13

is

an

example

of

a

pipe

and

filter

system

used

for

processing

invoices.

When

used

Commonly

used

in

data

processing

applications

(both

batch-

and

transaction-based)

where

inputs

are

processed

in

separate

stages

to

generate

related

outputs.

Advantages

Easy

to

understand

and

supports

transformation

reuse.

Workflow

style

matches

the

structure

of

many

business

processes.

Evolution

by

adding

transformations

is

straightforward.

Can

be

implemented

as

either

a

sequential

or

concurrent

system.

Disadvantages

The

format

for

data

transfer

has

to

be

agreed

upon

between

communicating

transformations.

Each

transformation

must

parse

its

input

and

unparse

its

output

to

the

agreed

form.

This

increases

system

overhead

and

may

mean

that

it

is

impossible

to

reuse

functional

transformations

that

use

incompatible

data

structures.

34

Chapter 6 Architectural design

An example of the pipe and filter architecture

35

Chapter 6 Architectural design

Application architectures

Application systems are designed to meet anorganisational

need.

As businesses have much in common, their applicationsystems also tend to have a common architecture thatreflects the application requirements.

A generic

application architecture is an architecture for atype of software system that may be configuredandadapted to create a system that meets specificrequirements.

Information and resource management systems are nowusually web-based systems where the user interfacesare implemented using a web browser.

For example,e-commerce systems are Internet-basedresource management systems that accept electronicorders for goods or services and then arrange delivery ofthese goods or services to the customer.

In ane-commerce system, the application-specific layerincludes additional functionality supporting a ‘shoppingcart’ in which users can place a number of items inseparate transactions, then pay for them all together in asingle transaction.

Chapter 6 Architectural design

46

Server implementation

These systems are often implemented as multi-tier clientserver/architectures (discussed in Chapter 18)

Models of application systems architectures help usunderstand and compare applications, validateapplication system designs and assess large-scalecomponents for reuse.

Transaction processing systems are interactive systemsthat allow information in a database to be remotelyaccessed and modified by a number of users.

Language processing systems are used to translatetexts from one language into another and to carry out theinstructions specified in the input language. They includea translator and an abstract machine that executes thegenerated language.